1. Field of the Invention
The invention concerns an axial flow turbine whose outlet rotor blades are followed by a diffuser, means for swirl removal from the swirling flow being provided within the retardation zone of the diffuser.
2. Discussion of Background
Such a turbine is known from EP-A 265 633. In order to meet the requirement of this patent for the best possible pressure recovery at part load, a straightening cascade is provided within the diffuser and this extends over the complete height of the flow duct. These means for removing swirl involve three aerodynamic ribs with thick aerofoil sections arranged evenly around the periphery. These aerofoil sections are designed from knowledge of turbomachinery and should be as insensitive as possible to oblique incident flow. The rib leading edges subject to incident flow are located relatively far behind the outlet edge of the last rotor blades in order to avoid excitation of the last row of blades due to the pressure field of the ribs. This distance is dimensioned in such a way that the leading edge of the ribs is located in a plane at which there is a diffuser area ratio of, preferably, three. The diffuser zone between the blading and the aerodynamic ribs should therefore remain undisturbed because of total rotational symmetry. The fact that no interference effects between the ribs and the blading are to be expected may be attributed to the fact that the ribs only become effective in a plane in which there is already a relatively low energy level.
In conventional gas turbines, the diffuser is subject to incident flow at a velocity ratio c.sub.t /c.sub.n of about 1.2 at idle, c.sub.t being the tangential velocity and c.sub.n being the axial velocity of the medium. This oblique incident flow leads to a reduction in the pressure recovery C.sub.p, as may be seen from FIG. 2, to be described later (curve A).
In other types of machine, such as for example steam turbines or gas turbines for fluidized bed firing, it is quite possible for the volume flow to be reduced to 40% so that c.sub.t /c.sub.n ratios of up to 3 occur. In such types of machines, the known diffuser configuration is not appropriate because the pressure recovery could even become negative, as may be seen in FIG. 2. This applies even in the case where the pitch/chord ratio of the aerodynamic ribs is 0.5 (curve A). Aerodynamic ribs with pitch/chord ratios of about 1 (curve B) cannot be used at all in such machines--even though they would give a somewhat larger pressure recovery at full load, i.e. at a c.sub.t /c.sub.n of about zero (see FIG. 2).
In addition to the large drop in pressure recovery, a strong vortex between the outlet rotor blades and the aerodynamic ribs is a characteristic feature under the extreme conditions mentioned. This is indicated in FIG. 1, which is also described later. The vortex is limited by the aerodynamic ribs on which the tangential component of the velocity is dissipated. If solid particles, in gas turbines for example, or water droplets, in steam turbines for example, are entrained in the resulting reverse flow, there may be acute danger of root erosion on the blades of the last rotor row.
In the case of gas turbines for fluidized bed firing, the pressure behind the blading, generally 0.98 bar at full load, can rise as high as 1.15 bar at 40% volume flow. This back pressure means that at 40% volume flow, significantly more drive power has to be provided for the machine than would be necessary in the presence of a satisfactorily operating diffuser.